1. Field of the Invention
The present invention relates to a effusion cell for Si which is used to obtain a sufficient quantity of Si molecular beam to produce a film in an MBE (molecular beam epitaxy) device, and a molecular beam epitaxy system using such effusion cell for Si.
2. Description of the Related Art
An MBE device is a device which applies a molecular beam of a proper material to a substrate heated in an ultrahigh vacuum to thereby produce a thin film of the material. Here, description will be given below generally of a molecular beam epitaxy system (MBE device) with reference to FIG. 2. In FIG. 2, the MBE device includes a manipulator 1 in the central upper portion thereof and the manipulator 1 supports a substrate 2, which faces downward. The manipulator 1 has a function to rotate the substrate 2 and a function to heat the substrate 2 An effusion cell 3 is used to radiate a molecular beam of a certain material toward the surface of the substrate. Actually, there are provided a plurality of effusion cells 3 which are arranged in a radial manner. The MBE device further includes a chamber 6 and shrouds 5 of liquid nitrogen which are disposed near to the wall surface of the chamber 6, while the liquid nitrogen shrouds 5 absorb gas to thereby prevent mutual heat interference. The chamber 6 can be made into an ultrahigh vacuum. The chamber 6 is connected with another vacuum chamber, while wafers can be replaced in such vacuum chamber. Since the effusion cell 3 is oblong or long lengthwise, it is easy to install four to eight effusion cells 3. When the MBE device is of a large size, ten or more effusion cells can be provided.
As a device which generates a molecular beam, normally, there is used a effusion cell 3 (K-cell; Knudsen cell). While the effusion cell 3 includes various modified versions, an example of the effusion cell 3 will be described in detail with reference to FIG. 4. In FIG. 4, there is provided a cylindrical crucible 8 with a bottom, while a heater 9 is disposed so as to enclose the crucible 8. This heater 9 is a ribbon-shaped heater or a coil-shaped resistance type heater. Further, outside the heater 9, there are disposed a plurality of cylindrical reflection plates (reflectors) 10. These reflection plates 10 are used to reflect the heat of the heater 9 and return it toward the crucible 8. The molecular cell 3 further includes a plurality of reflection plates 11 on the bottom surface thereof as well. The reflection plates, each of which is formed of a thin tantalum (Ta) plate, are superimposed on top of one another.
Below the crucible 8, there is disposed a disk-shaped support plate 12, and the outer-most ones of the side surface reflection plates 10 are connected to the support plate 12. The crucible 8 includes in the top portion thereof a flange portion 13 which is supported by the top ends of the reflection plates 10. In particular, the crucible 8 is supported by the support plate 12 through the reflection plates 10. The support plate 12 is also connected through a prop 14 (see FIG. 1) to an ultrahigh vacuum flange 15 which is disposed further downward. The ultrahigh vacuum flange 15 is mounted onto the flange 16 of the chamber 6 by a screw. If the screw is removed, then the ultrahigh vacuum flange 15 can be take out clean.
A thermocouple 17 is so disposed that the tip end thereof is in contact with the bottom portion of the crucible 8. While a material solid is to be charged into the crucible, the material solid is heated in a vacuum and is thereby turned into material liquid 18. The material liquid 18 evaporates in a liquid state and turns into a molecular beam. Depending on the kind of material, however, there is a case that it turns (that is, sublimes) from a solid state directly to a gaseous state without passing through a liquid state. For example, materials such as arsenic (As), phosphorus (P) and the like cannot take the form of a liquid state and, therefore, they are sublimed and are thereby turned into molecular beams. A crucible often uses PBN (Pyrolytic Boron Nitride) as the material thereof.
Since PBN has high resistance against heat and does not react on many substances, PBN is the most suitable for the material of a crucible. It is believed that nothing is better than PBN as a molecular beam crucible. This is the reason why PBN is used as the material of a crucible for most materials. An effusion cell is composed of an oblong PBN crucible 8, a resistance heating type heater 9 for enclosing the PBN crucible 8, and a plurality of cylindrical reflection plates 10. The arrangement shown in FIG. 2 is an ordinary molecular beam epitaxy system in which a plurality of effusion cells shown in FIG. 4 are provided upwardly and obliquely. With use of this molecular beam epitaxy system, substances such as Ga, As, In, P, Al, B and the like can be flown.
Si presents a problem in an effusion cell. Because Si has a high melting point (1410.degree. C.), it does not melt easily. Also, molten Si is extremely reactive. When a thin film formed of GaAs or InP and Si is added in a minute quantity as a dopant, in the case of Si, there is used an effusion cell using a conventional PBN crucible. In GaAs, Si may become an n-type impurity or a p-type impurity according to conditions. That is, Si is important as the dopant. When Si is used as a dopant, since the flux (the beam dose per unit time) of a molecular beam may be small, Si is not melted but Si, as it is in a solid state, is sublimed into a molecular beam. Because Si is held in its solid state and is not melted, even a crucible of PBN can hold Si therein. That is, although the cell temperature is lower than the melting point of Si, a slight amount of molecular beam can be generated. In other words, if Si is added in a slight amount as an impurity, then the above-mentioned structure using the PBN crucible can achieve its expected object.
However, there has come up a case which requires a stronger Si flux; for example, a case in which a Si thin film is formed. Here, let us describe three new demands: Firstly, there arises a demand in which a Si/Ge mixed crystal film is used as the material of a light emitting element. Since Si is a semiconductor of an indirect transition type, it is difficult for Si to emit light. However, it is said that Si can be turned into a direct transition type if it is mixed with Ge to thereby produce a mixed crystal, and thus the Si/Ge mixed crystal film has been studied as the material of the light emitting element that can be substituted for GaAs or the like. When producing a mixed crystal film, the molecular beam requires a fairly high intensity. Referring now to a second demand, it is expected that, if a Si thin film is formed uniformly on a Si wafer and a DRAM is formed on such Si thin film, then the resultant Si film may be better in quality than a Si film which is formed directly on a Si wafer. In this case, because a uniform Si film is formed, a strong flux is necessary. Such a weak molecular beam as flies away an impurity takes too much time and thus it is not practical.
As a third demand, a Si film is attached to a GaAs wafer and a new active element is produced on this Si film. This is a new and hopeful trial.
The above-mentioned cases are respectively trials in an experimental stage and require a strong Si molecular beam. To enhance the intensity of the Si molecular beam, it is also necessary that Si is melted into a liquid state and the melted Si liquid is then caused to evaporate. The melted Si liquid is strongly reactive and thus reacts on the crucible to thereby denature the material of the crucible. There is no available receptacle which can be so strong and heat resistant that can bear the melted Si liquid of high temperatures. In a PBN crucible which is normally used in the MBE, there arises a problem that it reacts on the melted Si liquid and the thus reacted material is taken into a growth film. Conventionally, there is no other way but to use Si itself as a receptacle. That is, part of a lump of solid Si is melted and the remaining solid portions of Si are used to hold the thus melted Si liquid therein. By the way, it is not possible for the resistance heating type heater to melt only the local portion of the solid Si.
Thus, there is used an electronic beam heating method (EB heating method) which is capable of heating the solid Si locally. Here, FIG. 3 shows a Si molecular beam source using an electronic beam heating method. A crucible 19 for an EB gun is a shallow receptacle, while a Si solid 20 is stored in this crucible 19. There is provided an EB gun which emits an electronic beam 21, while an electronic beam emitted from the EB gun is curved by a magnetic field and is thereby caused to collide with the upper surface of the Si solid 20. The thus collided electrons lose their kinetic energy to thereby generate heat here. The solid Si is partly melted due to the thus generated heat. Because the area of the solid Si with which the electronic beam is collided is small, the remaining portions of the solid Si still remain solid. Since the electronic beam is so guided as to collide just with the upper surface of the solid Si, the solid portions of Si provide a shape like a pond. Accordingly, the melted Si liquid 22 can be stored in this pond. And, the periphery of the pond is the solid Si 20 and thus it can be held by the crucible 19 formed of metal. There is no possibility that the metal crucible 19 can be invaded or affected by Si.
As described above, to produce a Si molecular beam having a large flux, there is used an electronic beam heating method. In FIG. 1, there is shown a general section view of a molecular beam epitaxial growth device using an effusion cell and an EB gun in combination. In a portion of the chamber 6, there is formed a laterally facing cylindrical portion, while a special flange 24 extending perpendicularly to the chamber 6 cylindrical portion is provided in front of the chamber 6 cylindrical portion; and, an ultrahigh vacuum flange 23 with an EB gun 4 mounted thereon is fixed to the flange 24. Further forwardly of the EB gun 4, there is disposed a crucible 19 for an EB gun and the Si solid 20 is stored in the EB crucible 19. An electronic beam is emitted from the EB gun 4, is curved by a magnetic field and is thereby collided with the top portion of the Si solid 20, so that, as shown in FIG. 3, a pond for the melted Si solid 22 is produced in the top portion of the Si solid 20. The molecular beam of Si is radiated from the melted Si liquid 22 toward the substrate 2.
Si is a metal which is high in the melting point and is very hard to handle. The PBN crucible cannot be used for Si and, for this reason, under the existing circumstances, there is no other way but to heat and fly Si using an electronic beam. In this case, such heating is executed locally and thus Si is locally melted into liquid. However, since the state of the electronic beam used is not stable, the quantity of the melted Si liquid varies all the time. Due to this, the quantity of the electronic beam varies with the passage of time and is thus not stable. While the thickness of the Si thin film is controlled on the basis of time, if the beam flux is unstable, then the thickness of the Si thin film is varied. The above-mentioned molecular beam epitaxial growth method is used to make a thin film such as a superlattice or the like to grow in a well controllable manner and, therefore, the thickness of the thin film must be controlled with accuracy. However, the above-mentioned local heating using the EB gun is unstable in this respect.
Also, in the electronic beam heating method, there exists a second problem. In particular, because the Si solid is struck by the electronic beam, part of the Si solid is ionized when it is flown. If such ionized Si attaches to the shroud, then the present portion of the shroud is charged with electricity. That is, due to such electrification, a flake (the attached material which is irregular in shape) can fly around to thereby contaminate a specimen.
Further, the electronic beam heating method raises another problem. Specifically, as can be seen from FIG. 1 as well, since the EB gun is mounted in the molecular beam epitaxy system in such a manner that it faces laterally, the EB gun occupies a wider space. When using a medium-sized molecular beam epitaxy system which is not so small as an ordinary PBN effusion cell, 8 pieces of PBN cells or so can be mounted. However, in the electronic beam heating method, only 4 pieces of EB guns or so can be mounted. And, when the PBN cells and EB guns are used together, a desired number of effusion cells cannot be mounted because the existence of the EB guns provides an obstacle to such mounting.
As described above, when Si is flown using the EB guns according to the electronic beam heating method, there are raised many problems. There still exists a strong requirement to fly Si by means of a normal effusion cell according to the resistance heating method using rather a crucible than the EB gun. In view of this, the present inventors have already invented a Si effusion cell which is very skillful. In particular, it is disclosed in Unexamined Japanese Utility Model Publication No. Hei. 3-116027 under the title of "A effusion cell for high temperature". In this application, a heater insulation member is formed of a sapphire, a crucible is formed of a sapphire, high-purity alumina and high-purity carbon, and a heater is formed of tungsten. Since the heater is held by the sapphire, even if Si is raised up to such high temperatures that can melt Si, there is no possibility that impurities can be flown around. On the other hand, when the crucible is formed of PBN, then nitrogen is flown to thereby contaminate a specimen. However, if the crucible is formed of a sapphire, then no nitrogen can be flown and the crucible can stand the high temperatures. Therefore, the whole of Si can be melted into liquid and the liquid Si can be held well.
Use of such crucible and resistance heating is very effective. However, since a large quantity of sapphires must be used, the effusion cell is very expensive as a whole. Although PBN is also expensive, when the sapphire is used as the material of the crucible and heater insulation member, then the resultant effusion cell is quite expensive as a whole. In view of this, there still exists a requirement to be able to obtain a device which not only is practical in cost but also is capable of turn Si into a molecular beam using a resistance heating method and a cylindrical crucible.